|Exploring for Oil and Gas Traps|
|Series||Treatise in Petroleum Geology|
|Part||Traps, trap types, and the petroleum system|
|Chapter||Classification of exploration traps|
|Author||Richard R. Vincelette, Edward A. Beaumont, Norman H. Foster|
Early concepts of traps
The earliest concept of a trap was made by William Logan in 1844 when he noted the occurrence of oil on anticlines. I.C. White took Logan's anticlinal trap concept and applied it to search for oil and gas in 1855. Since then, models and applications of trap concepts have evolved as new trap types have been discovered.
A trap consists of a geometric arrangement of permeable (reservoir) and less-permeable (seal) rocks which, when combined with the physical and chemical properties of subsurface fluids, can allow hydrocarbons to accumulate.
Three main trapping elements comprise every subsurface hydrocarbon accumulation:
- Trap reservoir—storage for accumulating hydrocarbons and can transmit hydrocarbons.
- Trap seal—an impediment or barrier that interferes with hydrocarbon migration from the reservoir.
- Trap fluids—physical and chemical contrasts—especially differences in miscibility, solubility, and density—between the common reservoir fluids (primarily water, gas, and oil) that allow hydrocarbons to migrate, segregate, and concentrate in the sealed reservoir.
Trap boundaries define the limits of the trap and usually consist of (1) boundaries between solids, such as the contact between reservoir and seal, or (2) boundaries between fluids, such as oil–water or gas–water contacts. Temperature can also control a trap boundary as displayed by gas hydrate traps. See Gas hydrates.)
Traps vs. accumulations
A trap may or may not contain oil or gas. Accumulations, or pools, are traps that contain oil or gas.
Subsurface conditions that impede oil or gas migration include the following:
- Capillary contrasts in pore throats in the seal vs. the reservoir
- Contrasts in physical/chemical properties of subsurface fluids (primarily oil, gas, and water)
- Rock/fluid chemical and physical interactions
Capillary contrasts are differences in the capillary properties of the pore-throat apertures of seal and reservoir rocks, generally caused by a difference in pore-throat aperture sizes. These capillarity contrasts commonly create the trap boundaries between reservoir and seal.
Trap closure is a measure of the potential storage capacity or size of the trap defined by the trap boundaries. Vertical closure is a measure of the maximum potential hydrocarbon column of the trap. Areal closure is a measure of the maximum area of the potential hydrocarbon accumulation within the trap boundaries. Volumetric closure integrates vertical and areal closure with pay thickness, porosity, and hydrocarbon saturation to provide the volume of the potential hydrocarbon accumulation within the trap boundaries.
A meaningful trap classification scheme must consider reservoir, seal, and fluid properties and how these properties relate to one another to form closure. Since closure is defined by trap boundaries, the proposed classification scheme is based on the geometry, composition, and genesis of trap boundaries.
- Classification philosophy
- Classification basis
- Classifying traps
- Trap systems: structural, stratigraphic, and fluidic
- Trap classification levels